AP PHOTOS: How fracking transforms fortunes, land

In this March 29, 2013 photo, a worker uses a headset and microphone to communicate with coworkers over the din of pump trucks, at the site of a natural gas hydraulic fracturing and extraction operation run by the Encana Oil & Gas (USA) Inc., outside Rifle, in western Colorado. The technique of hydraulic fracturing is used to increase or restore the rate at which fluids, such as petroleum, water, or natural gas can be recovered from subterranean natural reservoirs. (AP Photo/Brennan Linsley)

RIFLE, Colo. (AP) -- Three hours west of Denver, across the Continental Divide, the Rocky Mountains begin the long transition into high desert plateaus.

This sparsely-populated land is dotted with ranches and small towns that were once local hubs for mining the rich minerals found under the earth.

But over the past few years, this town and others have become increasingly a local center for the natural gas industry. Off the highway outside town in all directions, one can see evidence, large and small, of the latest local energy boom, from natural gas extraction all the way up the chain to refining.

Hydraulic fracturing — "fracking," for short — pumps millions of gallons of water mixed with fine sand and chemicals deep into oil and gas wells.

The water splits open oil- and gas-bearing rock. Specially formulated fracking fluids help carry the sand into the newly formed fissures and keep the cracks propped open.

The rapid growth of the oil industry in the region has brought opposition from those who warn of environmental costs. In some places the practice has been blamed for air pollution and gas leaks that have ruined well water. But federal and many state regulators say the practice is safe when done properly.